Water oxidation catalyst including cobalt molybdenum

ABSTRACT

A process for oxidizing water using hydrated cobalt molybdenum is disclosed. A plurality of hydrated cobalt molybdenum nanoparticles are supported on an electrode and are able to catalytically interact with water molecules generating oxygen. The catalyst can be used as part of an electrochemical or photo-electrochemical cell for the generation of electrical energy.

FIELD OF THE INVENTION

The invention relates to a process and apparatus of using cobaltmolybdenum as a catalyst for the electrochemical andphotochemical-electrolysis of water, and in particular to a process andapparatus using hydrated cobalt molybdenum as a catalyst for thephotochemical-oxidation of water.

BACKGROUND OF THE INVENTION

Hydrogen has long been considered an ideal fuel source, as it offers aclean, non-polluting alternative to fossil fuels. One source of hydrogenis the splitting of water into hydrogen (H₂) and oxygen (O₂), asdepicted in equation (1).

2H₂O→O₂+2H₂  (1)

In an electrochemical half-cell, the water-splitting reaction comprisestwo half-reactions:

2H₂O→O₂+4H⁺+4e ⁻  (2)

2H⁺+2e ⁻→H₂  (3)

and hydrogen made from water using sunlight prospectively offers anabundant, renewable, clean energy source. However, the oxygen evolutionhalf reaction is much more kinetically limiting than the hydrogenevolution half reaction and therefore can limit the overall productionof hydrogen. As such, efforts have been made to search for efficientoxygen evolution reaction (OER) catalysts that can increase the kineticsof OER and increase the production of hydrogen from water. Inparticular, oxides of ruthenium and iridium have previously beenidentified. However, as they are among the rarest elements on earth, itis not practical to use these catalysts on a large scale. Therefore,improved OER catalysts would be very useful in the development ofhydrogen as an alternative fuel source.

SUMMARY OF THE INVENTION

In one aspect there is disclosed a process for oxidizing water toproduce oxygen. The process includes placing water in contact withhydrated cobalt molybdenum, the hydrated cobalt molybdenum catalyzingthe oxidation of water and producing oxygen. The hydrated cobaltmolybdenum can be a plurality of hydrated cobalt molybdenumnanoparticles which may or may not be attached to an electrode with anelectrical potential applied between the electrode and the water togenerate oxygen.

In a further aspect of the invention, a cell for oxidizing water toproduce oxygen is disclosed. The cell comprises water and hydratedcobalt molybdenum, where the hydrated cobalt molybdenum catalyzes theoxidation of water and produces oxygen. The cell may further comprise acontainer to hold the water.

In yet a further aspect, a photo-sensitizer may be added to the water incontact with hydrated cobalt molybdenum and the water plus hydratedcobalt molybdenum plus photo-sensitizer mixture exposed toelectromagnetic radiation. In this aspect, the photo-sensitizer providesan electrical potential between the hydrated cobalt molybdenum and thewater. In some instances, the photo-sensitizer may be aruthenium-tris(2,2′-bipyridal) compound, such asruthenium-tris(2,2′-bipyridal) chloride.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic representation of the proposed mechanism by whichcobalt molybdenum (CoMoO₄) serves as a water oxidation catalyst forconverting water to its elemental components with electrons transferredto a sacrificial electron acceptor such as [Ru(bpy)³]³⁺ and S₂O₈ ²⁻;

FIG. 2 is a pair of scanning electron microscopy (SEM) images ofhydrated CoMoO₄ nanoparticles and anhydrous CoMoO₄ particles;

FIG. 3 is an EDX plot of the hydrated CoMoO₄ catalyst material;

FIG. 4 is an XRD plot of hydrated CoMoO₄ nanoparticles and anhydrousCoMoO₄ particles;

FIG. 5 is a graphical representation of cyclic voltammetry traces usinga scan rate of 5 mV/s for CoMoO₄-carbon paste electrode for one cycle,after 10 cycles and for a CoWO₄ electrode after 10 cycles;

FIG. 6 is a graphical representation FIG. 7 is a plot of theelectrochemical performance including the over potential versus thecurrent density for hydrated CoMoO₄ and anhydrous CoMoO₄;

FIG. 7 is a graphical representation of percent oxygen produced as afunction of time for a working potential applied to an ITO electrodecoated with hydrated CoMoO₄ catalyst material;

FIG. 8 is a graphical plot of TGA data for hydrated CoMoO₄.

DETAILED DESCRIPTION OF THE INVENTION

The present disclosure provides a method, apparatus and/or catalystcomposition for the oxidation of water to generate oxygen gases. Themethod includes providing a hydrated cobalt molybdenum (CoMoO₄) catalystmaterial and adding the catalyst to water under a condition effective togenerate oxygen. In one embodiment, the method further includes exposingthe water, which contains the catalyst, to light radiation to generateoxygen gases.

A “catalyst” as used herein, means a material that is involved in andincreases the rate of a chemical electrolysis reaction (or otherelectrochemical reaction) and which itself, undergoes reaction as partof the electrolysis, but is largely unconsumed by the reaction itself,and may participate in multiple chemical transformations. A catalyticmaterial of the invention may be consumed in slight quantities duringsome uses and may be, in many embodiments, regenerated to its originalchemical state. The reaction may include a water oxidation or oxygenevolution reaction.

In one aspect a water oxidation catalyst or an oxygen evolution catalystincludes hydrated cobalt molybdenum that splits water into oxygen andhydrogen ions.

In a further aspect there is disclosed an electrode for electrochemicalwater oxidation splitting water into oxygen and hydrogen ions thatincludes a substrate and an active material in contact with thesubstrate. The active material includes hydrated cobalt molybdenum.

In one aspect, the hydrated cobalt molybdenum may be combined withconductive particles such as carbon black and may also include a bindersuch as NAFION®, a sulfonated tetrafluoroethylene based fluoropolymercopolymer sold by DuPont. The combined material may be attached to anelectrode substrate using any method known to those in the art. Variouselectrode substrates may be utilized that are capable of conductingcurrent such as for example, glassy carbon, carbon black or othermaterials.

The catalyst can include a plurality of hydrated cobalt molybdenumnanoparticles. In some instances, the nanoparticles are uniform in sizeand can have an average particle size of less than 100 nm. In oneembodiment, the hydrated cobalt molybdenum is attached to an electrodeusing any method known to those in the art. For example for illustrativepurposes only, absorption techniques, adhesives, deposition techniquesand the like can be used to attach the hydrated cobalt molybdenum to theelectrode.

In some instances, the electrode can have channels and water can bebrought into contact with the catalyst at a rate that allows the waterto be incorporated into the electrode channels. In addition, theelectrode can be in an aqueous solution and/or be part of anelectrochemical cell and/or part of a photo-electrochemical cell, whichmay or may not include a container.

The container may be any receptacle, such as a carton, can or jar, inwhich components of an electrochemical device may be held or carried. Acontainer may be fabricated using any known techniques or materials, aswill be known to those of ordinary skill in the art. The container mayhave any shape or size, providing it can contain the components of theelectrochemical device. Components of the electrochemical device may bemounted in the container. That is, a component, for example, anelectrode, may be associated with the container such that it isimmobilized with respect to the container, and in some cases, supportedby the container.

In some instances, an electrochemical cell containing an embodiment ofthe present invention offers a highly efficient method of splittingwater using solar illumination, without the need for an appliedpotential. Upon oxidation of water at a photo-anode, hydrogen protonsare generated which are then reduced to form hydrogen gas at a counterelectrode. In addition, the oxygen and hydrogen generated from the cellcan be passed directly to a fuel cell to generate further power.

In a further embodiment, the electrochemical cell can be driven eitherby a photo-anode such as a dye sensitized semiconductor or an externalpotential. The dye sensitized semiconductor acts as achemical/photo-electrical relay system. For example and for illustrativepurposes only, FIG. 1 illustrates a sequence of electron transfer thatcan occur in a photo-electrical relay system. Examples of such relaysystems include ruthenium N-donor dyes such as ruthenium polypyridaldyes that can absorb visible light and accept electrons from a hydratedcobalt molybdenum catalyst material and thereby assist in the oxidationof water that is in contact with the catalyst. In some instances, thephoto-sensitizer can be a ruthenium-tris(2,2′-bipyridyl) compound suchas ruthenium-tris(2,2′-bipyridyl) chloride.

The invention is further described by the following examples, which areillustrative of specific modes of practicing the invention and are notintended as limiting the scope of the invention defined in the claims.

EXAMPLES Example I Preparation of Hydrated CoMoO₄

Starting materials of Co(NO₃)₂.6H₂O (Mw=291.03 g/mol) and Na₂MoO₄.2H₂O(Mw=241.95 g/mol) were purchased from Sigma-Aldrich and used directlywithout further purification. In a typical synthesis a (0.5M) Na₂MoO₄solution was added drop-wise into a (0.5M) Co(NO₃)₂ solution with strongagitation. Following the reaction, the solution mixture was rinsed withwater on a centrifuge and the particles were then washed with ethanolprior to drying overnight in an oven at 35° C. The final product was apurple powder material.

A final powder product was examined by SEM as shown in FIG. 2. It can beseen in the Figure that the hydrated CoMoO₄ particles had a smaller sizein comparison to the anhydrous particles. The powder particles were alsosubjected to energy dispersive x-ray (EDX) analysis as depicted in FIG.3 with an average particle size of less than 100 nm confirmed along withthe presence of cobalt, molybdenum, and oxygen. The analysis indicatesCobalt:Molybdenum ratio of 1:1 and a CoMoO₄:H₂O ratio of 1:4.

XRD data as shown in FIG. 4 is shown for hydrated CoMoO₄ and anhydrousCoMoO₄. The data indicates that a CoMoO₄:H₂O ratio may be from 1:1 to1:3 for the hydrated COMoO₄ material.

Example II Cyclic Voltammetry (CV) of CoMoO₄

Carbon paste electrodes were prepared by grinding CoMoO₄ nanoparticlesproduced according to Example I above with carbon paste (BASI, CF-1010).The CoMoO₄-loaded carbon paste was then loaded onto an electrode body(BASI, MF-2010) and sanded to produce a working electrode. Alternativelythe electrodes were produced by combining CoMoO₄ particles as producedabove with carbon black using Nafion as a binder material. The materialwas then drop cast onto a glassy carbon electrode.

The CV studies were performed in a simple 3-electrode cell with Ag/AgCland Pt wire as reference and counter electrodes, respectively. Theelectrolyte had a pH of 8 and was obtained with a phosphate buffer atconcentrations of 50 and 200 mM. Typical scan rates were 5 and 25 mV/s.

Cyclic voltammogram traces for the CoMoO₄ particles after one cycle, 10cycles and for CoWO₄-loaded carbon paste electrodes are shown in FIG. 5.As shown in the figure the hydrated CoMoO₄ particles after 10 cycleshave an increased performance at an applied potential of 1.1 Vincomparison to particles of CoMoO₄ particles on a first cycle andparticles of CoWO₄ following 10 cycles.

Tafel plot measurements, as shown in FIG. 6 of hydrated and anhydrousCoMoO₄ show that hydrated CoMoO₄ has significantly better performanceper unit of surface area than anhydrous CoMoO₄ under the same conditionsand applied overpotential. The performance characteristics of thehydrated CoMoO₄ at a pH of 8, a desirable range of pH for anelectrochemical cell for splitting water indicates an improvedelectrochemical catalyst for splitting water than may be produced in alarge scale using a hydrothermal reaction.

Example III Deposition of CoMoO₄ on Indium Tin Oxide Electrode

Indium tin oxide (ITO) electrodes were selected for additional wateroxidation testing with ITO glass slides measuring 25×75 mm purchasedfrom SPI Supplies (#6415-CF). ITO electrodes were produced by cutting anITO glass slide into four equal pieces using a diamond blade, each ITOglass slide producing four ITO electrodes.

To immobilize or attach hydrated CoMoO₄ nanoparticles on an ITOelectrode, cobalt molybdenum nanoparticles produced according to ExampleI were first dispersed in ethanol. A typical dispersion solutioncontained 10 mg of CoMoO₄ nanoparticles in 1 ml of ethanol. Thedispersion solution was sonicated for approximately 20 minutes and theCoWO₄ nanoparticles remained well-dispersed for up to several days inthe ethanol.

Deposition of CoMoO₄ nanoparticles onto ITO glass slides was performedusing a dipping technique and a Nima Dip Coater in order to achieveuniform coating. The CoMoO₄-ITO electrodes were then baked in an oven at150° C. for one hour.

Example IV Verification of Oxygen Production

An air-tight H-cell was designed to quantify oxygen production in thecell. A copper rod with an alligator clip was attached at one end tohold the CoMoO₄-ITO electrode, while Ag/AgCl and platinum coils wereused as reference and counter electrodes, respectively. The two chambersin this H-cell were typically filled with 35 ml of pH 8 phosphate buffer(200 mM). The electrode area was controlled by covering the undesiredarea with Teflon tape. The typical electrode area used in these studieswas 1 cm² and scan rates of 5 and 25 mV/s were used for a potentiostaticstudy in which real-time monitoring of the concentration of dissolvedoxygen in the electrolyte was performed. The study included a CoMoO₄-ITOworking electrode being set at voltages between 0.8-1.3 V versus theAg/AgCl reference electrode for 15 minutes under each applied potential,and the concentration of oxygen near the electrode was recordedcontinuously throughout the study. As shown in FIG. 7, an increase inoxygen concentration was observed at voltages as low as 1.0 V (˜200 mVoverpotential) with subsequent increases in the oxygen concentration athigher over potentials of 300 and 400 mV. A drop in oxygen concentrationwas also observed when there was no potential applied. Without beingbound by theory, this result suggests that Co²⁺ ions might have been“activated” to Co³⁺ or Co⁴⁺ for catalytic oxidation of water.

Example V

Referring to FIG. 8, there is shown a plot of the TGA analysis of thehydrated CoMoO₄ particles. The Total weight loss due to water by TGA wasfound to be approximately 10% with an atomic ratio of CoMoO₄:H₂O to beapproximately 1:1.

In one aspect, the atomic ratio of CoMoO₄:H₂O may be from 1:1 to 1:4 assupported by the examples.

The invention is not restricted to the illustrative examples describedabove. Examples described are not intended to limit the scope of theinvention. Changes therein, other combinations of elements, and otheruses will occur to those skilled in the art. The scope of the inventionis defined by the scope of the claims.

Having described our invention, we claim:
 1. A process for oxidizingwater, the process comprising: providing hydrated cobalt molybdenum;providing water; and placing the water into contact with the hydratedcobalt molybdenum, the hydrated cobalt molybdenum catalyzing theoxidation of water.
 2. The process of claim 1, wherein the hydratedcobalt molybdenum is a plurality of hydrated cobalt molybdenumnanoparticles.
 3. The process of claim 2, further including applying anelectrical potential between the hydrated cobalt molybdenum and thewater.
 4. The process of claim 2, further including adding aphoto-sensitizer to the water and exposing the water withphoto-sensitizer to electromagnetic radiation, the photo-sensitizerproviding an electrical potential between the hydrated cobalt molybdenumand the water.
 5. The process of claim 4, wherein the photo-sensitizeris a ruthenium-tris(2,2′-bipyridyl) compound.
 6. The process of claim 1,wherein the hydrated cobalt molybdenum has a CoMoO₄:H₂O ratio of from1:1 to 1:4.
 7. A cell for oxidizing water, said cell comprising: waterand hydrated cobalt molybdenum in contact with said water; said hydratedcobalt molybdenum catalyzing the oxidation of water.
 8. The cell ofclaim 7, wherein said hydrated cobalt molybdenum is a plurality ofhydrated cobalt molybdenum nanoparticles.
 9. The cell of claim 7,further including an electrical potential applied between said hydratedcobalt molybdenum and said water.
 10. The cell of claim 9, furtherincluding an electrode, said hydrated cobalt molybdenum attached to saidelectrode.
 11. The cell of claim 7, further including a photo-sensitizerin said water and an electromagnetic radiation source operativelyarrange to expose said water with said photo-sensitizer toelectromagnetic radiation, said photo-sensitizer providing an electricalpotential between said hydrated cobalt molybdenum and said water. 12.The cell of claim 11, wherein said photo-sensitizer is aruthenium-tris(2,2′-bipyridyl) compound.
 13. The cell of claim 7,wherein the hydrated cobalt molybdenum has a CoMoO₄:H₂O ratio of from1:1 to 1:4.
 14. A water oxidation catalyst splitting water into oxygenand hydrogen ions comprising hydrated cobalt molybdenum.
 15. The wateroxidation catalyst of claim 14 wherein the hydrated cobalt molybdenumhas the formula: CoMoO₄.
 16. The water oxidation catalyst of claim 41wherein the hydrated cobalt molybdenum includes a plurality ofnanoparticles having a size of less than one micrometer.
 17. The wateroxidation catalyst of claim 14 further including conductive particlesand a binder combined with nanoparticles of hydrated cobalt molybdenum.18. The water oxidation catalyst of claim 14 wherein the hydrated cobaltmolybdenum has a CoMoO₄:H₂O ratio of from 1:1 to 1:4.
 19. An electrodefor electrochemical water oxidation splitting water into oxygen andhydrogen ions comprising: a substrate; an active material in contactwith the substrate, the active material including hydrated cobaltmolybdenum; wherein water is split into oxygen and hydrogen ions. 20.The electrode of claim 19 further including conductive particles and abinder combined with nanoparticles of hydrated cobalt molybdenum. 21.The electrode of claim 19 wherein the hydrated cobalt molybdenum has aCoMoO₄:H₂O ratio of from 1:1 to 1:4.